Termination for an ultrasonic transducer



8, 1969 R. B. STELTING 3,475,633

TERMINATION FOR AN ULTRASONIC TRANSDUCER I Filed Feb. 12, 1968 SIGNALSOURCE LEGEND EACH TRANSITION BETWEEN THE SOLID AND DASHED --I LINES (28OR 36) REPRESENTS A I80 PHASE SHIFT INVENTQR RAYMOND B. STELTING BY W 0,W

ATTORNEY United States Patent 3,475,633 TERMINATION FOR AN ULTRASONICTRANSDUCER Raymond B. Stelting, Needham Heights, Mass., assignor toHewlett-Packard Company, Palo Alto, Calif., a corporation of CaliforniaFiled Feb. 12, 1968, Ser. No. 704,765 Int. Cl. H04r 17/10 US. Cl. 310--88 Claims ABSTRACT OF THE DISCLOSURE One face of a transducer element isacoustically coupled to a matched termination having a characteristicimpedance greater than that of the surrounding medium. The terminationcauses cancellation by phase interference of an acoustical wavegenerated by the transducer element and reflected back to the transducerelement from the interface between the end of the termination and thesurrounding medium. I

BACKGROUND AND SUMMARY OF THE INVENTION This invention relates toultrasonic transducers and, more specifically, to an improvedtermination for use therewith.

One face of a damped transducer element is typically terminated in alossy backing material having a characteristic impedance that matchesthe characteristic impedance of the transducer element. The terminationis made sufficiently long (about an inch or more) that an acousticalwave reflected from the interface between the end of the termination andthe surrounding medium is attenuated enough to prevent it from having anappreciable etfect upon the transducer element. This either prevents thereflected acoustical wave from being detected by the transducer elementor makes it readily distinguishable from other acoustical waves detectedby the transducer element.

It is the principal object of this invention to reduce both the lengthof the termination and the net effect upon the transducer element of theacoustical wave reflected back to the transducer element from theinterface between the end of the termination and the surrounding medium.This object is accomplished according to the preferred embodiment ofthis invention by providing a drastically shortened termination (aboutfifty thousandths of an inch long) having a characteristic impedancevery much greater than that of the surrounding medium. Notches areformed in the end of the termination so that one half of an acousticalwave transmitted from the transducer element to the interface betweenthe end of the termination and the surrounding medium is reflected fromthe interface one more time than the other half and so that both halvesare reflected back to the transducer element along paths of equal lengthin time. The effect upon the transducer element of one half of thereflected acoustical wave is therefore cancelled by that of the otherhalf.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing,which is a perspective view of an ultrasonic transducer according to thepreferred embodiment of this invention, there is shown a transducerelement such as a piezoelectric crystal 10. Electrically conductivemetal electrodes 12 and 14 are bonded to opposite and parallel faces ofthe piezo-electric crystal 10. A source 16 of electrical signal forenergizing the piezoelectric crystal is connected to the electrodes 12and 14. Upon being energized the piezoelectric crystal 10 generates oneacoustical wave that propagates in the forward direction 18 and anotheracoustical wave that propagates in the reverse direction 20. Atermination 22 made of a lossy material is bonded to the electrode 14 toattenuate the acoustical wave that propagates in the reverse direction20. This termination 22 is only about fifty thousandths of an inch long.It may be described as an acoustic transmission line having acharacteristic impedance Z that is matched to the characteristicimpedance of the piezoelectric crystal 10 and that is very much greaterthan the characteristic impedance Z of the surrounding medium, forexample, arr.

The reflection coeflicient I of the interface 24 between the end of thetermination 22 and the surrounding medium is given by where P is themagnitude of the acoustic wave generated by the piezoelectric crystal 10and incident upon the interface 24 (hereinafter referred to as theincident acoustical wave), and P is the magnitude of the acoustic wavereflected from the interface 24 back to the piezoelectric crystal 10.The magnitude P of the acoustic wave transmitted through the interface24 and into the surrounding medium is given by Since Z is very muchlarger than Z I is approximately equal to minus one. This indicates thatthere is one hundred and eighty degrees phase shift and almost onehundred percent reflection of each portion of the incident acoustic waveeach time it is reflected from the interface 24. The incident acousticwave may therefore be cancelled by phase interference if one half of theincident acoustic wave is reflected from the interface 24 one more timethan the other half and if both halves are reflected back to thepiezoelectric crystal 10 along paths of equal length in time.

Accordingly, the termination 22 is provided with flat end portions A andnotched end portions B that are alternately arranged so that the flatend portions A are collectively positioned directly opposite half of thepiezo electric crystal 10 and so that the notched end portions B arealso collectively positioned directly opposite half of the piezoelectriccrystal. The flat end portions A all lie in a reference plane 26 that isparallel to the faces of the piezoelectric crystal 10 and that normallyintersects the sides of the termination 22. One half of the incidentacoustic wave is therefore reflected once with a minimum reduction inamplitude and a one hundred and eighty degree phase shift from theinterface 24 back to the piezoelectric crystal 10, as generallyindicated by the lines 28. All portions of this half of the incidentacoustical wave travel along paths of equal length in time and thereforearrive back at the piezoelectric crystal 10 at the same time.

Every notched end portion B includes a pair of adjoining notches 30.Each pair of adjoining notches 30 has a flat side 32 that makes an angleof ninety degrees with the reference plane 26 and has a pair of flatsides 34 of equal length that intersect one another at an angle ofninety degrees at the reference plane. One of these sides 34 makes anangle of forty-five degrees with the side 32, and the other makes anangle of forty-five degrees with one side of the termination 22 or withanother flat side 32 that makes an angle of ninety degrees with thereference plane 26. The other half of the incident acoustic wave istherefore reflected twice with a minimum reduction of amplitude and athree hundred and sixty degree phase shift from the interface 24 back tothe piezoelectric crystal 10, as generally indicated by the lines 36.Every portion of this half of the acoustical wave travels along a pathof the same length in time as that traveled by every portion of theother half of the acoustical Wave. Thus, both halves of the incidentacoustic wave arrive back at the piezoelectric crystal 10 at the sametime. The effect upon the piezoelectric crystal 10 of one half of theincident acoustic wave is therefore cancelled by that of the other halfsince each half has substantially the same magnitude and is one hundredand eighty degrees out of phase with the other half. This prevents thepiezoelectric crystal 10 from detecting the incident acoustic Wave. Evenif the effect upon the piezoelectric crystal 10 of the incident acousticwave is not completely cancelled the detected incident acoustical waveis so reduced in magnitude as to be readily distinguishable from otherdetected acoustical waves.

In every case the dimensions of the fiat and notched portions A and B ofthe termination 22 must be large enough to be good reflectors.Furthermore, the characteristic impedance Z of the termination 22 mustalways be greater than the characteristic impedance Z of the surroundingmedium. When, as in the preferred embodiment described above, Z is verymuch greater than Z there is approximately one hundred percentreflection from the interface 24. Substantially equal areas of thepiezoelectric crystal 10 must then be positioned directly opposite theflat and notched end portions A and B of the termination 22. However,when Z is not very much larger than Z there is less than one hundredpercent reflection from the interface 24. In this case the areas of thepiezoelectric crystal that are positioned directly opposite the fiat andnotched end portions A and B of the termination 22 must be adjusted inorder to cancel, by phase interference, the portions of the incidentacoustical wave that are reflected back to the piezoelectric crystal 10.

I claim:

1. An ultrasonic transducer comprising:

a transducer element;

a termination acoustically coupled to said transducer element, saidtermination having a characteristic impedance greater than thecharacteristic impedance of the adjacent medium, said termination havingan end forming an interface with the adjacent medium, said end includingindented regions arranged so that one portion of an acoustical wavegenerated by said transducer element is reflected from said interfaceone time more than the remaining portion and so that both portions arereflected back to the transducer element along paths of substantiallyequal length in time.

2. An ultrasonic transducer as in claim 1 wherein:

said transducer element has a pair of opposite and parallel faces;

said termination is acoustically coupled to one of the faces of saidtransducer element;

said end of the termination includes flat regions lying in a referenceplane parallel to the faces of said termination; and

each of said indented regions comprises a notched region.

3. An ultrasonic transducer as in claim 2 wherein each of said indentedregions comprises a pair of adjoining notches, said pair of adjoiningnotches having a first fiat side that makes an angle of ninety degreeswith said reference plane and having second and third fiat sides ofsubstantially equal length that intersect one another at an angle ofninety degrees at said reference plane, said second side making an angleof forty-five degrees with said first side and said third side making anangle of forty-five degrees with a fourth side that is oriented at anangle of ninety degrees with respect to said reference plane.

4. An ultrasonic transducer as in claim 3 wherein said fourth sidecomprises one of a side of said termination and a side of said pair ofadjoining notches.

5. An ultrasonic transducer as in claim 4 wherein:

said notched regions are collectively positioned directly opposite halfof said transducer element; and

said fiat regions are also collectively positioned directly oppositehalf of said transducer element.

6. An ultrasonic transducer as in claim 5 wherein:

said transducer element comprises a piezoelectric crystal having a firstelectrode :bonded to one of said pair of opposite and parallel faces andhaving a sec ond electrode bonded to the other of said pair of oppositeand parallel faces; and

said termination is bonded to said second electrode.

7. An ultrasonic transducer as in claim 6 wherein said terminationcomprises an acoustically lossy material and has a characteristicimpedance matched to that of said piezoelectric crystal.

8. An ultrasonic transducer as in claim 7 wherein said flat regions andsaid notched regions are alternately arranged across the end of saidtermination.

References Cited UNITED STATES PATENTS 2,448,352 8/1948 Carlin 3109.1 X2,649,550 8/1953 Hardie et al. 3108.2 2,683,821 7/1954 Rocha 3108.22,753,543 7/1956 Rymes 3408 X 3,059,130 10/1962 Robins 310-82 X3,174,120 3/1965 Brouneus 310--8.3 X 3,283,264 11/1966 Papadakis 3109.5X 3,325,779 6/1967 Supernaw et al. 34010 X 3,357,641 12/1967 Martner3108.3 X 3,403,271 9/1968 Lobdell et al 310---9.1 X

5 MILTON O. HIRSHFIELD, Primary Examiner M. O. BUDD, Assistant ExaminerU.S. Cl. X.R.

